1. Field of the Invention
The present invention relates generally to a process for the recovery of usable economically valuable products, including a relatively pure iron or direct reduced iron product feedstock and, optionally, an iron oxide and an iron-carbon residual, from industrial waste streams typically comprising zinc compounds and iron compounds. A waste materials stream typically comprising zinc compounds and iron compounds, such as electric arc furnace (EAF) dust, is subjected to a combination of steps including leaching (digesting), resulting in a precipitate comprising iron oxides (an iron cake or IC), which then is subjected to roasting, resulting in an enriched iron compound (an enriched iron cake or EIC) which can be used as a feedstock for steel mills. The EIC typically is rich in direct reduced iron (DRI). Preferably, the precipitate containing iron oxides is removed from a process for the recovery of zinc oxide and zinc metal from industrial waste streams. During the recovery process, carbon compounds can be added to the waste stream, and a cake product is produced from the undissolved iron and carbon compounds, which also can be used as a feedstock for steel mills.
The present invention also relates to an enhanced recycling process which utilizes iron-rich materials produced by the invention as a feedstock ultimately to a steel mill. The iron-rich materials are fed to a reduction furnace in which the iron-rich cake is reduced to a higher purity iron product which may be fed a steel mill as the feed. Fumes exhausted from the reduction furnace can be processed by a baghouse or/and by a wet scrubber and the captured materials are then recycled to the present process where they are used in the recovery process of the invention. Fumes emanating from the reduction furnace contain particulate matter, and may include potentially valuable zinc, cadmium, and lead constituents. The fumes are filtered in a baghouse, either at the steel mill's baghouse or at an independent baghouse. The filter cake, which is an iron-poor mixture, may be combined with the initial waste feed (such as EAF dust) to the present process and/or other iron-rich materials, and processed according to this invention.
An alternative method of removing the particulate matter from the reduction furnace fumes is the use of a wet scrubber. A primary embodiment of the alternative recycle of the present invention is to pass the reduction furnace fumes through a recirculated water wet scrubber, such as a venturi scrubber. The fume constituents soluble in water will be removed from the fumes by the recirculated water. The loaded recirculated water then may be introduced to the ammonium chloride leach step. Alternatively, the wet scrubber can use an ammonium chloride solution instead of water. The particulate matter soluble in ammonium chloride, such as for example zinc, cadmium, and lead constituents, will be removed in the ammonium chloride solution in the wet scrubber. The loaded ammonium chloride solution then can be combined with the leaching step discussed above, which preferably is an ammonium chloride leach, resulting in an exceptional increase in the recycle of waste streams from, for example, the steel making process.
2. Prior Art
Industrial waste streams typically contain components which have economic value if they can be recovered in an economic fashion. For example, U.S. Pat. No. 3,849,121 to Burrows, now expired but which was assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out and discard the iron oxides and waste metals, so that the resulting zinc-compound-rich solution can be further treated to recover the zinc compounds.
Waste metal process dust typically has varying amounts of iron, lead, cadmium and other metals, in various forms, contained in the dust. The first step in the Burrows patent is the treating of the EAF dust with an ammonium chloride solution to leach any zinc oxide, lead oxide and cadmium oxide present in the dust into solution, without any leaching of the iron oxides present. The second step in the Burrows process is cementation in which the solution obtained from the initial leach is filtered to remove any remaining solids and then zinc dust is added. The third step in the Burrows patent then takes the filtrate from the cementation process and cools the filtrate and obtains what are called zinc oxide crystals. The Burrows patent does not teach the treatment or recovery of any values from the discarded iron oxide containing precipitates.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium. However, Peters does not disclose a method for further treating the removed components not containing zinc compounds.
Thus, there exists a need for a method which will allow the recovery of an iron product from industrial waste streams which can be subjected to further treatments, resulting in a relatively pure iron product, such as direct reduced iron, which can be used as the feedstock for other processes. The industrial waste streams of most interest for this invention include a typical electric arc furnace waste stream and the particulate matter filtered or otherwise removed from various substeps of the invention or the steel mill, particularly from the fumes of a reduction furnace, such as for example a rotary hearth furnace. Producing an iron product with a minimum amount of impurities, such as zinc ferrite, is advantageous because the iron product can be used as the feedstock for steel production processes. A method which results in the recovery of an iron product has additional value in that the iron product can be sold for use in other processes. Furthermore, recovery and retreatment of exhaust and other waste products from the present invention and from other processes and subprocesses has a beneficial effect on the environment, and a beneficial, economic effect on the cost of the steel making process.
Iron is smelted, or refined, in a furnace in which iron ore, coke and limestone are heated. Scrap iron also can be used as a feed to the iron smelting furnace. Prior to introducing scrap iron to the furnace, it is de-scaled of iron oxide, or rust. The mill scale, as it is called, is a waste product typically disposed of and not used in the iron production process. Steel is basically an iron alloyed with other chemical elements. Scrap steel also can be used as a feed in the making of steel. Mill scale also is not used in the steel production process. Finding an economical and/or beneficial use for this mill scale would provide iron and steel mills an opportunity to dispose of the mill scale. Likewise, used batteries provide a waste disposal problem. Used batteries also are not typically used in the steel making process. Rather than disposal in a landfill, it generally is preferable to recycle the used batteries, which are rich in iron oxide. Finding an economical and/or beneficial use for used batteries would reduce the quantity of such material sent to landfills and provide a recycle for usable components. All of these iron oxide rich materials can be added to the waste stream feed which is fed into the present process.
As can be seen, there exists a need for a method which will allow exhausts and fumes from reduction furnaces or the like to be filtered in a baghouse or/and a wet scrubber so that the filtrate can be recycled back to the leaching step of the recovery process. This need is addressed by the present invention.